Rotate array - by using another array

Question

I am trying to solve problem 189 on leetcode

The question is:

Given an array, rotate the array to the right by k steps, where k is non-negative.

According to the solution, I have tried out to implement the idea given in approach 2: by using an "extra" array. Still, I am unable to get the desired result. What's the issue with this code?

I tried brute force approach as well but it's not accepted there, so trying this!! Any help would be appreciated!

i = 0
l = len(nums)
arr = [0]*l

while i < len(nums):
    split = l-k
    if i >=0 and i < split:
        arr[i+k] = nums[i]
    if i >= k+1 and i < l:
        arr[i-k-1] = nums[i]

    i+=1

nums = [0]*l

for a in range(0,l):
    nums[a] = arr[a]

print(arr)
print(nums)

After getting some help, I tried with the slicing approach(as suggested in the comments) and this is the code I could come up with:

l = len(nums)

a = nums[:l-k] # appending part
nums = nums[l-k:] # real array

for i in a:
    nums.append(i)


print(nums)

Still, this is not running on that website. I am getting the correct output array but not exactly what's the requirement.

Hello there.. May be you can post the problem also from that site? That way, I think the question will be complete and clear. — Romeo Sierra
– Romeo Sierra, Commented Feb 10, 2020 at 1:07
I think you are overcomplicating things a bit with the rotation. Without spoiling the solution I'd suggest to try it using the modulo operator — DJSchaffner
– DJSchaffner, Commented Feb 10, 2020 at 1:44
I'd agree with @DJSchaffner.. One approach is to consider modulo operator. Also, you can consider python array slicing.. — Romeo Sierra
– Romeo Sierra, Commented Feb 10, 2020 at 1:47
I would recommend list slicing for this. That is simpler than creating a temporary list and copying elements. Think about where the front should start and append the tail from the start of the list. — jignatius
– jignatius, Commented Feb 10, 2020 at 2:43
@RomeoSierra thanks for the help. I tried with the slicing method but still confused. Can you take a look at the code? — Sanya
– Sanya, Commented Feb 10, 2020 at 4:27

Romeo Sierra · Accepted Answer · 2020-02-10 04:45:41Z

1

Check the below out. I came up with the correction needed.

from typing import List

def rotate(nums: List[int], k: int) -> None:
    l = len(nums)

    a = nums[:l-k] # appending part
    # nums = nums[l-k:] # real array # -> How you have written
    nums[:] = nums[l-k:] # real array # -> How it should be


    for i in a:
        nums.append(i)


    # print(nums) # Not needed


nums = [1,2,3,4,5]

rotate(nums, 3)

print(nums)

Problem with you code is that, the task expects you to modify the list that is being passed into the function. However when you assign it like nums = nums[l-k:], that will be visible only to within the function. That's why when you print it you saw the expected result. But that assignment will not modify the list referred to by that variable. Instead, you should do the assignment like nums[:] = nums[l-k:], in order to modify the list, which is in the global scope.

answered Feb 10, 2020 at 4:45

Romeo Sierra

1,7862 gold badges19 silver badges39 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Sanya Over a year ago

Interesting! it worked!! But I still did not understand. What exactly is nums[:] doing here? What's the difference between nums[:] and nums?

Romeo Sierra Over a year ago

I have tried to find some reference for us to read on that. But unfortunately I couldn't find any. If I find one, I will post here for sure.. :)

Collectives™ on Stack Overflow

Rotate array - by using another array

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related